Theoretical linguistics is that branch of linguistics that is most concerned with developing models of linguistic knowledge. Part of this endeavor involves the search for and explanation of linguistic universals, that is, properties all languages have in common. The fields that are generally considered the core of theoretical linguistics are syntax, phonology, morphology, and semantics. Phonology is often informed by phonetics, which like psycholinguistics and sociolinguistics is based on experimentation and therefore often not considered part of theoretical linguistics.